Discussion of the complex thermo-mechanical behavior of Ba0.5Sr0.5Co0.8Fe0.2O3−δ

The measurement of the thermo-mechanical properties of cubic Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) with ring-on-ring test and depth-sensitive micro-indentation testing revealed an anomaly between 200 °C and 400 °C. In particular, the stiffness decreases strongly between room temperature (RT) and 200 °C, and subsequently increases again to a stable value at ∼300 °C. Fracture stress and toughness show a qualitatively similar behavior. The fracture stress decreases up to 400 °C, before recovering between 500 °C and 800 °C. XRD verified that BSCF remains cubic in the relevant temperature range. Hence the anomalies are attributed to the transition of Co3+ spin states, which is experimentally confirmed by susceptibility measurements. The fracture surfaces of the specimens were not affected by the anomaly between RT and 700 °C, and only a purely transgranular mode was observed.